Does Varicella Vaccine REALLY Have to Be -15°C or Colder to Protect Patients from Disease?

Background:
Merck's Varivax®, the only varicella vaccine available in the U.S., must be stored at or below -15°C. There is currently no research showing that varicella vaccine stored above recommended temperatures, but still below freezing, does not produce immunity.
In 2006, The Oregon Immunization Program (OIP) discovered that a private clinic had consistently stored varicella vaccine above the recommended temperature range, but below freezing, for a total of 17 months.
OIP recommended recall and revaccination of patients who received varicella vaccine during this time, and that a study be conducted to titer for varicella immunity in these patients. Results could add to existing knowledge on varicella vaccine immunogenicity when stored outside of the recommended temperature range.

Setting:
A private family practice clinic in Oregon's Willamette Valley.

Population:
One hundred children who received improperly stored varicella vaccine.

Project Description:
A list of affected patients was pulled from ALERT, Oregon's Immunization Information System. Clinic staff mailed letters to patients, notifying them of the need for revaccination. Patients who returned for revaccination were informed of the study. For those who consented, blood samples were collected on-site and sent to the Oregon State Public Health Laboratory for analysis.
Thirty patients returned for revaccination. Only two consented to participate in the study.

Results/Lessons Learned:
Because of low participation in the study, no conclusions can be made about whether improperly stored varicella vaccine confers immunity.
The main barrier to participation was the need for venipuncture. The need to complete multiple forms was not considered a barrier to participation.
Before attempting a similar study again, it will be critical to consider parental motivation to allow their child to participate in research studies requiring venipuncture. Studies of this nature might be more appropriate in adolescent or adult populations, who may be better able to weigh benefits and costs.